The present invention relates to optical heads for use in data recording and retrieval systems.
Optical heads produce a focused beam of light on a medium containing information and detect the light reflected from the medium to determine the information content of the medium. Mechanisms for maintaining the focus and tracking of the optical head are required. With the recent advances in semiconductor lasers, there has been an increasing use of these lasers in data retrieval and recording systems. The compact audio disc player is a significant example of how lasers are used in playing back prerecorded music, which is a form of information. The concept of the compact audio disc player or the long play video disc player can be applied to the storage of data for a large computer network, mini computers or even personal computers.
When lasers are employed in these devices, the light emitted by the lasers must be controlled by appropriate optical components to produce a very small spot of light on the medium surface. Light reflected off of the medium is projected back to a detector from which recorded information and other signals relating to the status of the focus and tracking can be derived. Some examples of patents covering optical systems for such applications are U.S. Pat. Nos. 4,486,791, 4,193,091, 4,135,083, 4,034,403, 3,969,573, 4,057,833, 3,962,720, West German Pat. No. 2,501,124 and U.S. Pat. No. 4,198,657.
In some compact disc players the optical unit is separated into two parts. The first part contains a laser and a collimating lens to produce a nearly collimated (parallel) laser beam. It also contains a beam-splitter to direct the return light reflected off the medium to a detector reading the recorded information. The second part contains a focusing objective lens and a mechanism for moving it up and down so that the focus spot is maintained on the medium surface.
In another version of these devices, a laser beam from the laser diode is directly imaged onto the medium by an obeective lens without the use of a collimater. In the return path, the light is imaged on a detector by a beam-splitter.
FIG. 1 shows one embodiment of a prior art optical read head for a compact disc player. The head consists of a laser pen 10 and a focusing and tracking actuator 12. The laser beam is focused on an information medium 14 at a spot 16. A laser beam 18 in the shape of an elliptical cone is emitted from the semiconductor laser diode 20. Laser beam 18 passes unchanged through a beam-splitter 22 to a collimating lens 24. Collimating lens 24 produces a substantially parallel beam of light 26 which impinges upon an objective lens 28. Objective lens 28 focuses beam 26 onto medium 14 at spot 16. The focusing of lens 28 is accomplished through the use of a magnetic coil 30 which moves objective lens 28 up and down with respect to medium 14. In addition, a tracking actuator 12 may move objective lens 28 radially along the direction of medium 14, which is typically a disc.
When the laser beam is returned or reflected off of medium 14, part of the beam is reflected by beam-splitter 22 and passes through a biprism or cylindrical lens 32 to a photodetector 34. Lens 32 produces a pattern on photodetector 34 which varies according to the focus of spot 16. Thus, when detector 34 detects a variation from the ideal focus, appropriate electrical signals can be supplied to coil 30 to move objective lens 28 to the correct position. This mechanism is somewhat complicated and requires a large number of elements which must be precisely aligned relative to each other.
FIG. 2 illustrates another embodiment of prior art optical head. A laser diode 36 emits a diverging laser beam 38 which passes through a beam-splitter 40 directly onto an objective lens 42. Again, lens 42 is mounted in a focusing and tracking actuator 44 which includes a coil 46. The beam impinges upon a medium 48 and a portion of the reflected beam is directed by beam-sllitter 40 through biprism 50 onto photodetector 52. This embodiment eliminates the need for collimating lens 24 of FIG. 1, but requires that the laser pen and focus and tracking actuator of FIG. 1 be combined in one unit because of the need to precisely align objective lens 42 and beam-splitter 40. Thus, the embodiment of FIG. 2 cannot be produced modularly as can the embodiment of FIG. 1. In the embodiment of FIG. 1, the use of collimater lens 24 obviates the need for precise alignment of focusing and tracking actuator 12 and laser pen 10. Thus, the disadvantage of the embodiment of FIG. 2 is that in the event of a malfunction the entire unit must be repaired or replaced.
A third prior art optical head utilizing a pai of hologram lenses is shown in FIG. 3. A hologram lens is a diffraction grating which was produced using holographic methods. A diffraction grating is a grating having a series of slits so that it diffracts light shined upon it. Light impinging on a diffraction grating will produce a series of diffracted beams at different angles from the central axis of the impinging light beam. The value of the angles of diffraction depend upon the wavelength of the light and the spacing of the grating. Diffraction gratings can be created mechanically, but there is a limit to the size of the spacing that can be achieved. A hologram lens is a diffraction grating created by the use of two interfering coherent laser beams in such a manner that the beams form a suitable angle relative to each other and a diffraction grating corresponding to the resulting interference pattern is formed. This interference pattern is projected onto a substrate, such as glass coated with photoresist. Upon development of the photoresist, unexposed areas (negative photoresit) or exposed areas (positive photoresist) are removed, leaving a number of parallel grooves. Vacuum deposition of a suitable metal on the grooves provides diffraction grating of the reflection type, comprising a number of equidistant parallel lines. A discussion of the formation of a hologram lens according to various techniques is set forth in U.S. Pat. No. 4,560,249.
The optical head of FIG. 3 uses a laser diode 54 to emit a laser beam 56. Laser beam 56 impinges upon a hologram lens 58, and one of the diffracted beams from hologram lens 58 impinges upon hologram lens 60. The diffracted beam from lens 58 to lens 60 is a parallel beam of light, and thus hologram lens 58 replaces the collimating lens of FIG. 1. This beam hits hologram lens 60 at an angle, causing hologram lens 60 to emit a focused beam onto a medium 62. Thus, hologram lens 60 replaces objective lens of FIGS. 1 and 2. On the return path, the undiffracted, collimated beam of light passing through hologram lens 58 impinges upon a photodetector 64 after passing through a biprism or wedge 66. The collimation of the return beam allows a photodetector 64 to be placed a different distance from medium 62 than laser 54. Photodetector 64 includes four separate photodetectors 68. Biprism 66 splits the laser beam to create two focused beams of light which fall on different ones of detectors 68. A change in focus will cause these beams of light to move from one of detectors 68 to another, thereby enabling the detection of an out-of-focus condition. The apparatus of FIG. 3 is disclosed in U.S. Pat. No. 4,458,980.
An alternative to a biprism lens is a cylindrical lens which is polished with two separate curvatures to produce an astigmatic beam. The astigmatic beam is focused on the center of a four quadrant photodetector and will be a circle when in focus. When out of focus in one direction, it will be an elliptical beam at a first angle and thus two of the photodetectors will detect more light than the other two, indicating a focus error. When out of focus in the other direction, an elliptical beam at a different angle is produced, which can also be detected.
In addition to correcting for focus error, or the distance from the objective lens to the medium, the optical head must also track the data. The data is typically written onto a series of concentric or spiraling grooves on a disc. The grooves are very narrow and are spaced by approximately 1.6 microns to allow the placement of pits having a size on the order of 1 micron. Data is typically stored in the form of a combination of pits and "lands," where lands are the area between the recessed pits. The pits serve to scatter the laser beam while the lands reflect it. A change in the amount ofrreflected light indicates a transition from a pit to a land. Often, it is these transitions which are used to represent bits of data rather than the pits and lands themselves.
Because the thin groove which the pits and lands are centered on is separated from other grooves by a distance of the same order of magnitude as the laser beam diameter, a diffraction effect is produced on the beam reflected back to the detector. This diffraction effect produces three beams which partially overlap. If the beam moves off the groove to the area between grooves, interferences of the overlapping beams causes the right and left half of the pattern to alternate in brightness. By using multiple photodetectors, this change in brightness can be monitored to detect tracking errors and produce a feedback signal to put the beam back on track.
Another method for tracking is the use of a diffraction grating in front of the laser to split the laser beam into three beams before it hits the medium. The center tracking beam is focused on the track with the left and right sides being on the left and right sides of the track. These three beams are reflected back and split off by a beam-splitter to a separate set of photodiodes which detect the intensity of the two weaker beams. When they are of different intensities, the error signal activates a servo mechanism that moves the optical head to correct for the error.
Another type of medium uses thermal magnetic recording to provide an erasing and rewriting capability. The principle of thermal magnetic recording is based on a characteristic of certain ferromagnetic material. When the temperature of the material is raised above the Currie temperature, the magnetization of the material can be affected by a small magnetic field. This principle is used for thermal magneto-optics data storage where laser beam is focused on the recording medium to raise the temperature of the medium above the Currie temperature. A small electro-magnet is placed on the other side of the meiium to create a magnetic field near the medium to change the magnetization of the medium. To retrieve information from the medium a laser beam is again focused on the medium but at lower power. Depending on the magnetization of the medium, the polarization of the beam reflected off the medium is either unchanged or rotated by about 0.4 degree. A polarizer inserted before a photodetector allows the detector to sense these two different states of polarization of the returned beam. One method of erasing the recorded information is to first reverse the direction of magnetization of the electro-magnet and then apply a focused laser beam to raise the temperature of the medium to above the Currie temperature to uniformly magnetize the medium in one direction.
To use the above principle in optical data storage systems an optical head is needed to produce a focused laser beam on the thermal magnetic medium. Moreover, a polarizer is needed to permit the detector to read the information recorded on the medium.
Polarizers needed for the thermal magnetic optical heads are available commercially in two forms. One is a sheet type polarizer based on dichroism, which is the selective absorption of one plane of polarization in preference to the other orthogonal polarization during transmission through a material. Sheet polarizers are manufactured from organic materials which have been imbedded into a plastic sheet. The sheet is stretched, thereby aligning the molecules and causing them to be birefringent, and then dyed with a pigment. The dye molecules selectively attach themselves to the aligned polymer molecules, with the result that absorption is very high in one plane and relatively weak in the other. The transmitted light is then linearly olarized. The optical quality of the sheet type polarizers is rather low. They are used mostly for low power and visual applications.
Another type of polarizer is based on the use of wire grid structures to separate the two orthogonal polarizations. When light radiation is incident on an array of parallel reflective stripes whose spacing is on the order of or less than the wavelength of the radiation, the radiation whose electric vector is perpendicular to the direction of the array is reflected. The result is that the transmitted radiation is largely linearly polarized. The disadvantage of both types of polarizers is that their light efficiencies are typically less than 30%.
There is thus a need for a simpler optical head with a more efficient polarizer.